docs: address copilot review feedback

Fix links in viewing-catalog.md and filter-catalog.md to point to
the new frontend system catalog-customization.md instead of the old
guide. Fix the Kubernetes entity content extension ID to use the
correct `entity-content:kubernetes/kubernetes` format. Wrap raw
extension definitions in `createFrontendModule` in search and
scaffolder docs. Correct TechDocs addon installation instructions
to explicitly install addon modules rather than claiming
auto-discovery.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

docs/features/search/getting-started.md

@@ -119,14 +119,24 @@ export const MySearchResultListItem = SearchResultListItemBlueprint.make({
 });
 ```
 
-Install this in your app by passing it to `createApp`:
+Install this in your app by wrapping it in a frontend module and passing it to `createApp`:
+
+```tsx title="packages/app/src/search/searchModule.ts"
+import { createFrontendModule } from '@backstage/frontend-plugin-api';
+import { MySearchResultListItem } from './MySearchResultListItem';
+
+export const searchCustomizations = createFrontendModule({
+  pluginId: 'search',
+  extensions: [MySearchResultListItem],
+});
+```
 
 ```tsx title="packages/app/src/App.tsx"
 import { createApp } from '@backstage/frontend-defaults';
-import { MySearchResultListItem } from './search/MySearchResultListItem';
+import { searchCustomizations } from './search/searchModule';
 
 const app = createApp({
-  features: [MySearchResultListItem],
+  features: [searchCustomizations],
 });
 
 export default app.createRoot();

docs/features/search/how-to-guides.md

@@ -178,14 +178,24 @@ The extension is then exported from your plugin's alpha entry point and
 automatically discovered when the plugin is installed.
 
 If you need to provide a search result list item extension from your app
-rather than a plugin, you can install it directly in `createApp`:
+rather than a plugin, wrap it in a frontend module and pass it to `createApp`:
+
+```tsx title="packages/app/src/search/searchModule.ts"
+import { createFrontendModule } from '@backstage/frontend-plugin-api';
+import { YourSearchResultListItem } from './YourSearchResultListItem';
+
+export const searchCustomizations = createFrontendModule({
+  pluginId: 'search',
+  extensions: [YourSearchResultListItem],
+});
+```
 
 ```tsx title="packages/app/src/App.tsx"
 import { createApp } from '@backstage/frontend-defaults';
-import { YourSearchResultListItem } from './search/YourSearchResultListItem';
+import { searchCustomizations } from './search/searchModule';
 
 const app = createApp({
-  features: [YourSearchResultListItem],
+  features: [searchCustomizations],
 });
 
 export default app.createRoot();